Hummocky cross-stratification is a type of

sedimentary structure Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particl ...

found in

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s. It is a form of

cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

usually formed by the action of large storms, such as

hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...

s. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of water below fair-weather

wave base The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. At water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion a ...

and above storm-weather wave base. They are not related to "

hummock In geology, a hummock is a small knoll or mound above ground.Bates, Robert L. and Julia A. Jackson, ed. (1984). “hummock.” Dictionary of Geological Terms, 3rd Ed. New York: Anchor Books. p. 241. They are typically less than in height and ...

s" except in shape.

History

The name was introduced by Harms et al. in 1975. Before this time, these structures were recognized under many different names. When hummocky cross-stratification was founded, it was originally given the name “truncated wave-ripple laminae,” by Campbell (1966, 1971).Surlyk, F., and N. Noenygaard. "Hummocky Cross-stratification from the Lower Jurassic Hasle Formation of Bornholm, Denmark." ''

Sedimentary Geology ''Sedimentary Geology'' is a peer-reviewed scientific journal about sediments in a geological context published by Elsevier Elsevier () is a Dutch academic publishing company specializing in scientific, technical, and medical content. It ...

'' 46.3-4 (1986): 259-73. Print. The main features were listed by Bourgeois (1980), Harms et al. (1982), and Walker (1983), in order to identify the structure. Dott and Bourgeois launched an idealized hummocky stratification sequence. From bottom to top, these include: first-order scoured base (± sole marks); characteristic hummocky zone with several second-order truncation surfaces separating individual undulating lamina sets; a zone of flat laminae; a zone with well-oriented ripple cross-laminae and symmetrical

ripple Ripple may refer to: Science and technology * Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid ** Ripple, more generally a disturbance, for example of spacetime in gravitational waves * Ripple (electri ...

forms; all overlain by a more or less burrowed mudstone or siltstone.Dott, R. H., and Joanne Bourgeois. "Hummocky Stratification: Significance of Its Variable Bedding Sequences." Geological Society of America Bulletin 93.8 (1982): 663-80. Print. Walker (1983) wanted to create a second sequence, but it was decided that this sequence offers the best basis for studying hummocky cross-stratification for the future.

Composition

This structure is commonly found in silt to fine sand. It is typically interbedded with bioturbated mudstone. It commonly contains

concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...

s of abundant mica and plant detritus in the tops of many laminae. This helps indicate a shape sorting. Although hummocky cross-stratification is usually found in shallow marine

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s, it has also been found in some lacustrine sedimentary rocks.

Common characteristics

In plan view (seen from above), it takes on the form of hummocks and swales that are circular to elliptical, with long wavelengths (1–5 m) but with low height (10s of centimeters). Laminations drape these hummocks; in cross-section view, these laminations have an upward curvature, and low angle, curved intersections. Hummocky cross-stratification can form in sediments up to about 3 cm in diameter, with near-bed water particle velocities between about 40–100 cm/s.

Formation of structure

This structure is formed under a combination of unidirectional and oscillatory flow that is generated by relatively large storm waves in the ocean. Deposition involves fallout from suspension and lateral tractive flow due to wave oscillation. As the large waves drape sand over an irregular scoured surface, this strong storm-wave action erodes the seabed into low hummocks and swales that lack any significant orientation. It is usually formed by redeposition below normal fair weather wave base delivered offshore by flooding rivers and shoals by large waves.

Depositional environments

During ancient times, hummocky cross-stratification was located in shallow marine environments, on the shore face and shelf by waves. It can also form on land during especially large storms when large amounts of water are pushed up onto the tidal flat. These landward deposits feature smaller bed forms due to the attenuation of storm waves as they move onto the land. While it is usually formed in marine settings by the action of storms (e.g.hurricane) it may also be deposited in fluvial strata; a fluvial origin is more likely if the unit solely comprises sand.

References